TrueWoW

Is anyone else not getting the hp bonus when switching into frost presence?

you probably have "Improved Frost Presence" in your talents
therefore you won't notice the change.

Improved Frost Presence: This talent now grants 4/8% stamina when the death knight is in Unholy and Blood Presences to match the 8% from Frost Presence.

It also gives you damage reduction done to you by 1%/2% when you have Frost presence
(meaning 9%/10% total with frost presence)